1. Field of the Invention
The present invention relates to a method for evaluating a photo mask for use in a semiconductor process and method for manufacturing a semiconductor device.
2. Description of the Related Art
In recent years, stricter dimensional accuracy of a photo mask has been rapidly required. For example, the dimensional uniformity in the photo mask area is required to be below 10 nm. In the manufacturing process of the photo masks, many more items are checked to decide whether manufactured photo masks are good or failure. If, therefore, even one of these checking items does not satisfy their specification, the manufactured photo mask is rejected as being failure. Under such a stricter accuracy thus sought, a high accuracy manufacturing technique has been progressed, but the manufacturing yield is becoming poor.
It is necessary to satisfy the specification of the photo mask in order to obtain a desired exposure latitude in a wafer exposure process where the photo mask is used In a conventional photo mask specification it has been determined that, even if respective checking items have their values all reach the limit value of the specification, it is possible to obtain a desired exposure latitude.
There is really a very rare case that all the checking items of a practical photo mask have their values reach the limit values of the specification. There are in reality more cases that, even if some checking items of the photo masks exceed the values of the specification, others fall within the values of the specification with some margin.
Such photo masks are treated as being failure. In this case, however, it is considered that, in them, there exist some photo masks capable of obtaining a desired exposure latitude, that is, some photo masks involving no real problem in their mass production.
The reason is as follows. That is, even in the case where some photo masks have their checking items exceed the specification value while others have their checking items fall within the specification value, if a decreased exposure latitude caused by checking items which exceeds the specification values is made smaller than an increased exposure latitude caused by checking items which fall within the specification value with some margin, then it is possible to obtain a desired exposure latitude as a whole.
By the way, there is JPN PAT APPLN KOKAI PUBLICATION NO. 2002-260285 as the prior art. This prior art shows a dimensional variation of the photo masks and their average value as well as the use of a phase shift mask merely described in connection with the transmittivity and phase difference. However, this is not adequate as to make a decision whether the photomask is good or failure. There is a risk that a good mask will be decided as being failure. This involves a fall in manufacturing yield of photo masks, thus presenting a problem.
Further, the dimensional variation now under consideration includes a global dimensional variance occurring on a whole area of a photo mask (reticule) and a local dimensional variation occurring at a very small area. Between them, a different influence is exerted on the exposure latitude.
In the conventional photo mask evaluation method, no distinction is given between the global variation and the local variation and the deteriorated amount of the exposure latitude is evaluated with the same extent of influence. In the case where the extent of difference of the local variation exerted on the exposure latitude is smaller (vice versa) than that of the global variation, overestimation is involved on the deteriorated amount of the exposure latitude. Therefore, there is a risk that a good photo mask will be decided as being an failure one. This involves a lower manufacturing yield of photo masks.